The monoxygenase system of the endoplasmic reticulum of liver cells can utilize a wide variety of substrates including cholesterol to form various bile acids, steroid hormones and drugs. There is as yet no clear understanding of the mechanisms by which so many substrates are utilized, or how the activities of specific reactions are regulated. It appeared to us that a careful description of the development of a normal metabolic process carried out by the monoxygenase system might serve as a foundation for the subsequent study of the isolated components of the system. We will study the development of bile acid systhesis in the chick embryo. Preliminary work has indicated that the kinds and amounts of bile acids in gall bladder bile are substatially different in the embryo and germ-free adult. We will describe the changing bile acid pattern during development, and the enzymatic basis for the changes. We will also determine whether chick embryo liver cells are able to synthesize bile acids during culture. Success here would provide the opportunity to examine the regulation of bile acid synthesis under conditions much easier to define and modify than in the intact organism. The direct actions of hormones would be studied, for example. Preliminary work has shown that enzymes of the monoxygenase system are maintained in chick embryo liver cells during culture, and can be further increased by adding phenobarbital to the medium.